This invention relates generally to processes and devices for conducting lyophilization or freeze drying procedures, and particularly relates to determining the end of a primary stage in a freeze drying procedure conducted in a tray-type device.
Freeze-drying devices are known that remove water from products by lyophilization or freeze drying procedures. In particular, tray freeze drying devices present a chamber in which trays of products are placed on shelves. Product can be contained in bottles or can be in bulk and loose in the tray. The trays rest on shelves in the chamber during the freeze drying process.
These tray-type freeze drying devices usually operate in three different stages. The first or initialization stage freezes the product to a low temperature such as -40.degree. Centigrade to insure that the product falls well below its eutectic temperature and that the entire solution becomes frozen. Next, a primary drying stage removes moisture from the frozen product by sublimation. This occurs by maintaining the temperature of the product below its eutectic temperature and encouraging the frozen water to leave the product in the form of vapor. The vapor becomes collected elsewhere on such as a condenser. This primary drying stage operates at moderate shelf temperatures and vacuums of substantially 800 milliTorr. The last or secondary drying stage removes the remaining one or two percent of water from the product by desorption; the device substantially reduces the pressure surrounding the product and increases the product temperature. This substantially removes the moisture remaining in the product.
Previously, the operator of a tray type drying device had no positive indication of the end of a primary stage and the beginning of a secondary drying stage. An inexperienced operator would conduct the primary stage for an extended period of from 12-24 hours to insure completion of the primary stage or attempt to view the product occasionally to see if the product looked dry. More experienced operators would maintain the shelf temperature constant and look for a rise in product temperature to indicate the end of the primary stage. The problem with this latter method is that a rise in product temperature could also mean that the product was melting. What is needed then is a positive indication of the end of a primary stage of freeze drying.